The present invention relates generally to adhesive compositions for securing a coating, such as a nonstick coating, to a substrate and, more particularly, to an adhesive composition or basecoat that can be applied directly onto either a treated or untreated substrate to secure one or more coatings to the substrate.
Fluoropolymer resins such as polytetrafluoroethylene resins (PTFE), fluorinated ethylene propylene resins (FEP), perfluoroalkoxy resins (PFA), or combinations thereof are known to have superior nonstick properties. Therefore, such resins are used in a variety of industries, including the cookware industry to minimize the sticking of food particles to the cookware. Due to the nonstick nature of fluoropolymer resins, there have been difficulties in making the coatings adhere well to the substrate.
Conventionally, adhesive resins or basecoats have been used in order to better adhere the fluoropolymer topcoat to the substrate. In general, such basecoats are a combination of a high temperature binder resin such as polyamideimide resins (PAI), polyethersulfone resins (PES) or polyphenylene sulfide resins (PPS) with a fluoropolymer resin. The performance of these basecoats depends upon a stratification of the applied coating. This stratification has resulted in a coating rich in high temperature binder on the bottom and rich in fluoropolymer at the top. The binder-rich bottom provides adhesion to the substrate while the fluoropolymer-rich top provides a layer to which subsequent fluoropolymer topcoats could be fused by sintering at high temperature.
The performance of such systems is at best a compromise. The bottom layer of these basecoats is not a purely binder resin. Therefore, roughening of substrates by mechanical (e.g. gritblasting) or chemical (e.g. etching) means is required to assist the basecoat in holding the topcoat onto the substrate. Also, considerable levels of fluoropolymer resins are required in the basecoat in order to provide a top layer sufficiently rich in fluoropolymer resin for good bonding of subsequent fluoropolymer topcoats. These fluoropolymer resins in the basecoat are disadvantageous because they detract from the adhesion of the basecoat to the substrate, and they produce soft and damage-prone basecoats due to the thermoplasticity of the fluoropolymer resin.
Efforts to overcome these deficiencies have included the addition of mica particles, ceramic fillers or metal flakes to the basecoat in order to increase the hardness of these basecoats. However, such additives do not improve adhesion of the topcoat to the substrate.
It has also been attempted to apply nonstick coating systems to untreated smooth substrates. In each of U.S. Pat. Nos. 5,049,437, 5,071,695, and 5,079,073, there is disclosed a coating composition comprising an undercoat applied to the substrate, a primer containing a perfluorocarbon resin applied to the undercoat, and a topcoat applied to the primer and comprising a perfluorocarbon resin. According to these patents, the adhesion of high melt viscosity fluoropolymer coatings to substrates is achieved through chemically induced stratification or formation of a concentration gradient in the primer. Nevertheless, the coating compositions disclosed in these patents exhibit the same problems as described above.
It is desired to provide a basecoat for nonstick coating systems in which the nonstick coating is firmly anchored in the basecoat, and the basecoat firmly adheres to a smooth or roughened substrate.